Retroreflectivity is a term that describes an object's ability to reflect a wave front such as light, electro-magnetic waves or other forms of radiation back to the source along a path or vector that is substantially parallel to the vector of the source wave front path.
Retroreflectivity is a phenomenon that may occur in nature. One example of natural occurring retroreflectivity is the ability of an animal's eyes to reflect light such that the eyes appear to glow. This retroreflectivity occurs because the eyes of most animals include a focusing lens and a partially reflective layer of tissue near or part of the retina that is positioned substantially near a focal plane of the focusing lens. As a result, the eyes of animals may reflect a portion of the light entering the eye back to the light source on a path that is substantially parallel to the light source path. As such, objects that include a lens and a surface having some degree of reflectivity that is positioned substantially near a focal plane of the lens, may be defined as a retroreflector.
Retroreflectors may also be manufactured. Some examples of artificial retroreflective instruments include reflective highways signs, bicycle reflectors and corner reflectors. One known example of a corner reflector is a surveyor's reflecting prism that may be used with a surveyor's total station to calculate a distance between the total station and the prism. Other retroreflector examples may be optical instruments that include a lens and a surface that has a degree of reflectivity that is positioned substantially near a focal plane of the lens. Examples of such optical instruments may be rifle scopes, binoculars and cameras.